Ceramic structures, usually and preferably multilayered, are used in the production of electronic substrates and devices. Many different types of structures can be used, and a few of these structures are described below. For example, a multilayered ceramic circuit substrate may comprise patterned metal layers which act as electrical conductors sandwiched between ceramic layers which act as insulators. The substrates may be designed with termination pads for attaching semiconductor chips, connector leads, capacitors, resistors, covers, etc. Interconnection between buried conductor levels can be achieved through vias formed by metal paste-filled holes in the individual ceramic layers formed prior to lamination, which, upon sintering, will become a sintered dense metal interconnection of metal-based conductor.
In general, conventional ceramic structures are formed from ceramic greensheets which are prepared by mixing a ceramic particulate, a thermoplastic polymer binder, plasticizers, and solvents. This composition is spread or cast into ceramic sheets or slips from which the solvents are subsequently volatilized to provide coherent and self-supporting flexible greensheets. After blanking, stacking and laminating, the green sheets are eventually fired at temperatures sufficient to drive off the polymeric binder resin and sinter the ceramic particulates together into a densified ceramic substrate.
The electrical conductors used in formation of the electronic substrate may be high melting point metals such as molybdenum and tungsten or a noble metal such as gold. However, it is more desirable to use a conductor having a low electrical resistance and low cost, such as copper and alloys thereof.
Current ceramic structures for electronic applications range up to about 127 mm. which can hold about 121 silicon devices. Typically, several of these structures are linked together through hardwire interconnections to perform, for example, a computing function. Inherently, there are propagation delays associated with such an arrangement due to the distances between linked ceramic structures. The electrical performance of the arrangement can be improved by decreasing the distance between ceramic structures.
One solution is to fabricate larger structures of 300 mm or more to hold more silicon devices. The surface area of such a ceramic structure is roughly 4 times that of existing ceramic structures. Since more of the silicon devices will be on a single structure, the inherent propagation delays associated with the hardwire interconnections will be eliminated. It is expected that vastly improved electrical performance will result.
The difficulty arises, however, in fabricating such large ceramic structures. According to conventional tape casting technology, all that need be done is to "simply" scale up existing tooling and technology to arrive at a larger ceramic structure. The cost, however, for such an endeavor would be prohibitive.
The present inventors have proposed joining several smaller ceramic structures into a single large ceramic structure. Aside from the present invention being a lower cost alternative to simply scaling up existing tooling, the proposed invention is also advantageous in that smaller ceramic structures have better dimensional stability than larger ones, thereby indicating that large ceramic structures comprised of smaller, joined ceramic structures will have better dimensional stability than tape cast large ceramic structures. The finished size of such a large ceramic structure would be about 300 mm or more to meet the future demands of electrical performance. This solution is not without difficulty either as the challenge is to join smaller greensheet segments together while maintaining electrical continuity across the entire ceramic structure and to be able to sinter the large ceramic structure without cracking and distortion.
Thus far, no solutions in the prior art have been proposed which meet this challenge.
The following references are relevant to the joining of ceramic parts in general.
Schroeder et al. U.S. Pat. No. 4,420,352 discloses the joining of ceramic heat exchanger parts wherein the adjacent surfaces to be joined are locally heated by RF heating. Ebata et al. U.S. Pat. No. 4,724,020 discloses a similar process but wherein the local heating is by high voltage torches.
Ferguson et al. U.S. Pat. No. 4,767,479 discloses a method of bonding green (unfired) ceramic casting cores. The casting cores are made up of ceramic particles and a binder. The binder in the casting cores is softened by applying a solvent and then ceramic filler particles are added to at least one of the surfaces to be joined. Thereafter, the ceramic casting cores are assembled until the solvent has evaporated.
Gat-Liquornik et al. U.S. Pat. No. 4,928,870 discloses the joining of ceramic parts by placing a metal foil or wire between the surfaces to be joined and then subjecting the metal foil or wire to a high current for a short period of time.
Iwamoto et al. U.S. Pat. No. 4,952,454 discloses the joining of ceramic parts wherein a paste including metals and metal oxides is applied to the surfaces to be joined and then the assembly is heated to effect bonding.
Ito et al. U.S. Pat. No. 4,953,627 discloses the joining of ceramic heat exchanger parts wherein a bonding material, e.g. glass, is applied to the surfaces to be joined and then the assembly is fired to effect bonding.
It is, accordingly, an object of the present invention to form a large ceramic structure for electronic applications from smaller electronic structures.
It is another object of the invention to form the large ceramic structure in a way to maintain electrical continuity between the joined segments.
It is yet another object of the invention to have a method for fabricating such a large ceramic structure.
These and other objects of the invention will become more apparent after referring to the following description considered in conjunction with the accompanying drawings.